Extending “Balanced Cross-Section” structural validation techniques to three dimensions has been an area of active research and development for two decades. Due to the inherent difficulty of the challenge, the resulting tools and methods have had limited practical application. We propose that the advent of so-called “Geological Knowledge-Oriented” earth models makes this traditionally difficult problem more practical.
Accurate depiction of subsurface structure is fundamental to petroleum exploration and development. Similarly, an accurate understanding of the kinematic development of structurers through geologic time is a key constraint to petroleum system and other modeling techniques based on application of physical principals. However, subsurface and deep-time analysis of geologic structures is highly under-constrained. Traditional “balanced sections” and restoration tools were developed to test the plausibility of interpretation based on fundamental concepts of mass-conservation (simplified to area or line-length conservation) and kinematic compliance (Dhalstrom, 1969, Boyer and Elliott, 1982, and Medwedeff and Suppe, 1997). Although natural variation in the mechanical properties of rocks is important, use of mechanical models in restoration is severely challenged by the inelastic, non-reversible, and non-linear rheology of rock strata.
Manual and computer-aided workflows for application of balancing concepts to cross section construction and analysis are widely applied in structural geologic analysis. These tools are effective in areas where the geologic deformation approximates plane-strain but are difficult to apply and much less predictive in areas with more general or more complex deformation.
Computer-aided approaches extending balancing constraints to mapped horizons and 3D have been under development for about twenty years (e.g. Geiser, et al., 1988, Durand-Riard, P., et al., 2010). Approaches that have been applied include:
vertical-shear flattening of fault blocks (Gratier and Guillier, 1993),
surface unfolding (Mallet, 2002; Chapter 8.5),
multiple-surface unfolding (Mallet, 2002; Chapter 8.6),
fault slip accommodated by vertical-shear (e.g. Clarke et al, 2006),
fault-parallel flow, and
mechanical (typically elastic) unfolding (e.g Guzofski et al., 2009 and U.S. Pat. No. 7,480,205 B2).
Although several commercial products have been or are being developed (Geosec-3D®; 3D Move®; KINE3D®, and Dynel3D®) none, in our view, have gained routine use for interpretation validation. Again, in our view, the reasons for this are a combination of (1) the limited flexibility to or (2) the great effort required to properly treat complex geometry and topology inherent in geologic structures for which the tools would be most useful. Mechanical solutions to restoration are additionally challenged by the need for specification of scale-appropriate, effective rheology parameters throughout the volume of interest. Such parameters are largely unconstrained.
There exists a need for improved restoration methods for complex subsurface models that will allow better seismic interpretation of potential hydrocarbon reservoirs.